Word of advice, guys: NEVER play Halo co-op over live. Campaign, Firefight or Spartan Ops; it's delay lag no matter how good a connection you have. Play it on a LAN or a split screen, or by yourself.

And Legendary is pretty easy as long as you play practically, and don't try to do things the fun and fancy way. That can be somewhat of a challenge, tho..
I played the first episodes on Legendary, and the instant respawning means it's just a matter of going in, getting some kills, then dying and repeating. They also shower you with power weapons all the time, so..